Add slash commands for adding context into the assistant (#12102)

Tasks

* [x] remove old flaps and output when editing a slash command
* [x] the completing a command name that takes args, insert a space to
prepare for typing an arg
* [x] always trigger completions when  typing in a slash command
* [x] don't show line numbers
* [x] implement `prompt` command
* [x] `current-file` command
* [x] state gets corrupted on `duplicate line up` on a slash command
* [x] exclude slash command source from completion request

Next steps:
* show output token count in flap trailer
* add `/project` command that matches project ambient context
* delete ambient context

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant/src/assistant.rs

@@ -7,6 +7,7 @@ mod prompt_library;
 mod prompts;
 mod saved_conversation;
 mod search;
+mod slash_command;
 mod streaming_diff;
 
 use ambient_context::AmbientContextSnapshot;
@@ -16,6 +17,7 @@ use client::{proto, Client};
 use command_palette_hooks::CommandPaletteFilter;
 pub(crate) use completion_provider::*;
 use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
+pub(crate) use prompt_library::*;
 pub(crate) use saved_conversation::*;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};